This invention relates to a scanning electron microscope and to a method of analysing a specimen by means of such a microscope, wherein the specimen is situated within a chamber which contains a gaseous medium.
The gaseous environment avoids or mitigates certain of the problems which arise from analysing a specimen in a high vacuum. For example, the gaseous environment may prevent or inhibit degradation of a biological specimen and can dissipate surface charges which would otherwise accumulate on a non conducting specimen to the detriment of image resolution.
In some electron microscopes, the gaseous environment is also used to amplify the secondary electron signal provided at the specimen. The secondary electrons obtained at the specimen as a result of the interaction with the scanning electron beam are accelerated towards an anode positioned generally above the sample. As the secondary electrons accelerate through the gaseous medium, they collide with gas molecules releasing further electrons which are also accelerated towards the anode. Since the electrons released by previous gas collisions can, in turn, release more electrons by further collisions in the gas, the acceleration of the original secondary electrons through the gas can trigger a cascade or avalanche of collisions which, in effect, provides the amplification.
European Patent No. EP-B1-330310 (Electroscan Corporation) and U.S. Pat. No. 4,785,182 (Mancuso et al) relate to examples of environmental scanning electron microscope (ESEM) which employ this gas amplification technique. Both microscopes detect the amplified secondary electron signal by measuring the current collected on the anodes used to accelerate the secondary electrons.
A paper of G D Danilatos published in 1992 (Proc. 50th annual meeting of the Electron Microscopy Society of America held jointly with the 27th annual meeting of the Microbeam Analysis Society and the 19th annual meeting of the Microscopical Society of Canada/Socixc3xa9txc3xa9de Microscopie du Canada) discusses the possibility of detecting the amplified secondary electron signal by collecting photons generated in the electron avalanche in the gas, but still uses a positive electrode positioned 3 mm above the specimen to provide the necessary electron acceleration for the avalanche to occur.
The performance of these microscopes can be adversely affected by the interactions between the primary electron beam, i.e. the beam emitted from the electron column onto the specimen, and molecules of gas in the chamber.
According to a first aspect of the invention, there is provided a scanning electron microscope comprising generating means for generating a scanning beam of electrons and for delivering the beam into a chamber for containing a gaseous medium, the chamber having a specimen holder for holding a specimen to be analysed, the holder being electrically insulated from the surrounding structure which carries the holder, means for applying a negative potential to the holder so as to generate an electric field within the chamber, the arrangement being such that, in use, the scanning beam impinges on the specimen so as to provide secondary electrons on the specimen surface, the electric field accelerating the secondary electrons in a direction away from the specimen surface and into a collision zone in the chamber where collisions between the accelerated secondary electrons and the gas molecules of the gaseous medium initiate a cascade of collisions, thereby generating an amplified secondary electron signal, the scanning electron microscope further comprising detecting means for detecting the secondary electron signal.
By applying a negative potential to the specimen holder, the need for a separate positive electrode above the specimen is avoided. Consequently, the working distance in the chamber can be minimised, which correspondingly reduces any scattering of the primary beam by the gaseous medium. It has been found with this configuration that working distances of 1 mm or less can be achieved.
In addition, it has been discovered that, unexpectedly, the application of a negative potential to the specimen holder appropriately modifies conditions at the specimen surface (giving rise to said electric field at the surface even if the specimen is non conductive). Thus, the microscope can obtain images of both conductive and non-conductive specimens.
It has also been found that the risk of breakdowns in the gas is also minimised which allows relatively large fields to be applied to the specimen holder. The large fields, in turn, give rise to improved gas amplification of the secondary electron signal.
Preferably, the detecting means is arranged to detect photons generated as a result of said cascade of collisions in the gas.
Since the microscope can be configured to have a small working distance, the collision zone can be of a correspondingly small volume. In such a case, the density of photons generated in the zone is relatively high, and this facilitates photon collection by a photo-sensitive device (for example a photo-multiplier). Thus, this type of detecting means is particularly suitable for the microscope.
Preferably, the detecting means includes photo collection means comprising a light pipe which extends into the chamber and, in use, gathers photons generated in the collision zone and directs those photons to a detector.
Additionally or alternatively, the photon collection means may to advantage comprise a mirror in the chamber.
Preferably, the mirror is curved (preferably paraboloidal) so as to focus the photons on the detecting means or another point of the collection means (for example the light pipe).
Preferably, the collection means comprises the light pipe and the mirror, the mirror being arranged so as to reflect photons towards the light pipe in order to increase photon detection.
Conveniently, the means for applying a negative potential to the holder comprises an electrical lead which extends into the chamber to connect the holder to a voltage source.
Preferably, the microscope includes further electrode means situated adjacent the portion of the detecting means which receives the photons to be detected, and means for applying a positive voltage to said further electrode means.
The application of the positive voltage to the further electrode means draws the secondary electrons and negatively charged particles released in said cascade of collisions, towards the detecting means and thus moves the collision zone closer to the detecting means.
The further electrode means may to advantage comprise an electrical conductor which is situated between the front of the detecting means and the collision zone and is so apertured as to allow photons to pass from the collision zone to the detecting means. Such a conductor may be in the form of a grid.
The grid is conveniently mounted on the detecting means from which the grid preferably extends towards the collision zone.
Where the detecting means includes a light pipe, the grid is conveniently mounted on the end of the light pipe and lies on a notional substantially hemispherical or part ellipsoidal surface.
The invention also lies in a scanning electron microscope comprising generating means for generating a scanning bean of electrons and for delivering the bean onto a specimen in a gaseous medium in a chamber, electrode means for accelerating electrons, released from the specimen as a result of the beam, into a collision zone and photo detecting means for detecting photons released in said collision zone, wherein the electrode means is situated adjacent the detecting means, preferably in the paths of photons from the collision zone to the detecting means.
According to a second aspect of the invention, there is provided a method of analysing a specimen by a scanning electron microscope, the method comprising the steps of generating a scanning beam of electrons, directing the beam into a chamber containing a gaseous medium, and including a holder carrying the specimen, applying a negative bias potential to the holder so as to provide an electric field within the chamber, the scanning beam impinging on the specimen and producing on the specimen surface secondary electrons which are accelerated by the electric field in a direction taking the secondary electrons away from the specimen surface and into a collision zone in the chamber where collisions between the accelerated secondary electrons and the gas molecules of the gaseous medium initiated cascade of collisions and detecting the resulting signal.
Preferably, said signal comprises detectable photons generated by said cascade of collisions.